This invention relates to a Fourdrinier-type paper machine and an apparatus for improving the operation of and reducing the waste produced by such paper machine. In particular, it relates to an apparatus to be added to the wet end of such a paper machine to prevent the wet pulp mixture from draining off the side of the fabric, without causing the usual large edge wave by providing continuous support, adjustable in amount along its length, for a curl in the edge of the fabric.
When the Fourdrinier Paper Machine was invented many years ago for making paper on a continuous basis a device was required to "hold" the mixture of fiber and water on the endless wire. A special "dam" was designed for that purpose and was called a "Deckle Strap". It consisted of an endless rubber strap, approximately 2 inches square in cross section by whatever length was needed--usually enough to travel with the Fourdrinier wire a distance of two thirds of the flat area length before returning to the starting point at the headbox. This quite adequately held the mixture of 1% fiber and 99% water on the Fourdrinier wire until enough water had been drained from the mixture for the sheet to solidify and form with no further danger of spillage over the edge, as long as lineal speeds were kept low.
When the machine speed was increased, however, it became difficult to keep the deckle strap on the supporting pulleys or "wheels". To overcome this problem the "Deckle Straps" were replaced by "Deckle Boards" fastened to the outlet of the headbox. The "Deckle Boards" were stationary and had to be sealed to the wire to prevent leakage. A rubber strip set in the botto of the "Deckle Board" provided the seal but it also caused additional problems. In spite of constant water lubrication between the moving wire and the stationery rubber strip, the wire or fabric wore and became unusable and stock rolled up into small balls or "dams" at the rubber seals. Each time the mixture came in contact with one of these small "dams", or obstructions, it literally threw the flow away from the edge in a wave-like pattern out toward the center of the forming sheet, causing the edge of the sheet to be thinner and with a slight ridge at the center of the wave. This clearly was detrimental to production as this entire area of the sheet could not be used and had to be trimmed off before the final processing.
As the machine speed and the cost of the fabrics increased it became obvious that some new method of containing the stock and water mixture on the fabric had to be devised. The paper mills started to use small wedge-shaped blocks, sometimes called "Curl Blocks", between the fabric and the drainage elements to raise the edge of the fabric to form a sort of a continuous "pocket". This worked quite well as far as fabric wear was concerned but did little or nothing to eliminate the edge wave problem and did not even completely prevent edge spillage. The blocks could not be placed close enough to the headbox to be effective and the fabric sagged between the blocks. Even though there were no obstructions in the form of rolled up stock "dams", the initial jet of pulp and water hit the fabric before it was fully turned up, or curled, to form the containing pocket and an edge wave resulted. Additional edge waves formed at each following block because of the sagging fabric. As speed increased, the placement of the blocks became more and more critical, if not impossible, and even greater edge waves formed. It was not at all unusual to have the basis weight of the edges of the sheet on the order of 20% to 25% less than the balance of the sheet width. The "bad" edges had to be trimmed off and production suffered.
The invention relates to improvements over the apparatus described above and to solutions to the problems raised thereby.